Adding a file completer class for positional or optional arguments that take a file.

Useful primarily to show only certain extensions. Based closely on default completer in __init__.
Minor fixes for FilesCompleterFiles Parser

Added missing subprocess import.
Added check for user forgetting to make input list a list and just entering a single string.
Added optional file listing, with trailing slashes supported.

Bugfix for failed calls. Now uses _wrapcall for simpler code.
Fixing global bash completion for other types of completion.
Merge pull request #29 from henryiii/master

FilesCompleter addition